Ethene is one of the most important basic raw materials in chemical industry, and is also one of the chemical products with the largest output in the world. It is the basic raw material for producing various organic chemical products and synthetic materials.
Presently, the industrial process for producing ethene worldwide mainly comprises cracking light oil or naphtha at high temperature. A large amount of petroleum is consumed every year, and the petroleum resources on the earth keep on shrinking day by day.
The proved global natural gas reserves are about 134×1012 cubic meters, and the number of recently discovered large natural gas fields keeps on increasing. If natural gas is adopted to replace petroleum for producing ethene, the pressure on petroleum demand can be greatly alleviated, and the development of natural gas chemical industry can be also benefited.
Methane is the major component (more than 90%) of dry natural gas; for purpose of convenience, sometimes methane is mentioned instead of natural gas from this point to the end of the present invention.
In worldwide laboratory studies, two conventional heating methods are adopted for converting methane into ethene, i.e. indirect method, and direct method. The indirect method comprises (1) ethene is generated from methane via methanol, which has the disadvantage of high energy consumption during gas generation; or (2) ethene is generated from methane via synthetic gas, in which the issue of how to inhibit regeneration of methane during reaction process has to be addressed.
The most important method in the direct method is oxidation coupling of methane. In 1982, G. E. Keller and M. M. Bhasin from UCC (US) published the result of ethene production through catalysis oxidation coupling of methane for the first time. Since then, researchers worldwide mainly focus on study in two aspects: (1) searching for high quality catalyst, (2) improving reactor. Y. Jiang, et al. believe that ethene should have single-pass yield above threshold value of 40% to make production of ethene from methane economically feasible. But until today, the ethene single-pass yield is still below 30%.
Therefore, production of ethene from methane through conventional heating method is still in research stage, and a method for achieving industrial production with good economic benefit is still unavailable.
Another method for producing ethene from methane is microwave chemical method, including plasma chemical method. The microwave chemical reaction has two significant advantages: (1) chemical reaction rate is greatly improved; (2) reaction that is hard to carry out from the point of view of conventional thermodynamics can be relatively easy to carry out. Methane is the organic molecule with most stable structure, and the reaction for producing ethene from methane through dehydrogenation coupling in absence of oxygen only can be carried out at high temperature of 1400° C. But with microwave chemical method, the methane can be easily decomposed into ethene. Although according to literature publications worldwide, ethene single-pass yield has already reached 30% which is higher than that of conventional heating method, the yield is still lower than 40% which is the internationally proved threshold value for economic feasibility.
Available microwave chemical experiments are mainly carried out in rectangular waveguide or resonant-cavity formed by the rectangular waveguide, which are still limited in rectangular waveguide; obstacles for producing ethene from methane (natural gas) by microwave chemical method comprises:
(1) Small resonant-cavity volume for microwave chemical reaction;
(2) Small flow rate of feed gas methane (natural gas);
(3) Ethene single-pass yield less than 40%.
Therefore, until today, the technical background is that no matter whether ethene is produced from methane (natural gas) by conventional heating method or microwave chemical method, the three obstacles including small cavity volume, low methane flow rate, and low ethene single-pass yield still need to be addressed. These obstacles, well known worldwide, have been intensively studied by numerous scientists for dozens of years, and are still unsettled.